Some prior art current measuring meters, i.e., ammeters, employ a pivoted polarized armature or motor magnet to which a pointer is connected. The motor magnet is mounted in operative relation to a current conducting bar and a polarized holding magnet is operatively disposed relative to the motor magnet to influence the motor magnet constantly toward a zero position. When current flows through the current bar, the current generated magnetic field reacts with the magnetic field of the motor magnet and causes the motor magnet to move from its zero position against the biasing or restoring force exerted by the holding magnet. Accordingly, the pointer will be caused to swing to indicate on an associated scale the magnitude of the current.
For an example of the foregoing type of current measuring meters, reference may be had to U.S. Pat. No. 2,867,768. As disclosed in this patent, a meter may be calibrated for various ranges by weakening or strengthening the holding magnet (varying the degree of magnetic saturation of the holding magnet) as by demagnetizing or remagnetizing the holding magnet. Accordingly, a meter may be calibrated for a high current range by fully saturating the holding magnet while an identically constructed meter may be calibrated for a lower current range by demagnetizing or remagnetizing the holding magnet to less than full saturation. Although this procedure is workable in theory, there are practical drawbacks including difficulty in precisely controlling the degree of partial saturation of the holding magnet in a manufacturing environment.
The above patent also illustrates two conventional types of mounting techniques for the motor magnet. According to one of these techniques, the motor magnet is mounted to a shaft which is journalled between two axially spaced bearings. This spaced bearing type of mount, however, makes meter fabrication and assembly relatively difficult and costly. Respecting the other mounting technique, the motor magnet is mounted on a concentric sleeve bearing having a central hole shaped at its bottom to form a jewel. The sleeve bearing is slipped over a bearing axle and the magnetic attraction between the motor magnet and holding magnet holds the sleeve bearing such that the jewel thereof effects point bearing contact with a pointed end of the axle.
The spaced bearing type of mount has been used successfully in both high current and low current measuring meters, i.e., those calibrated for high full scale current values and low full scale current values, respectively. On the other hand, the sleeve bearing mount has found practical application in only high current measuring meters, i.e., meters calibrated for full scale current values of 30 amps or more. Prior attempts to use a sleeve bearing mount in low current measuring meters have resulted in inadequate meter performance. Generally, the sleeve bearing mount cannot provide as low a frictional resistance to turning of the motor magnet as can be achieved by a spaced bearing mount. Accordingly, a higher torque is required to overcome such frictional resistance and this presents a major problem when measuring small currents. Still another problem is that when the holding magnet is calibrated for low current measurement by weakening its magnetic field intensity, the magnetic attraction between the holding magnet and motor magnet generally is insufficient to hold the sleeve bearing in proper axial position on the bearing axle with the result being higher frictional resistance to turning of the motor magnet. Accordingly, it heretofore has been necessary to use the more difficult and expensive spaced bearing type of mount in low current measuring meters.